Optimization of Microwave Assisted Extraction Conditions to Improve Phenolic Content and In Vitro Antioxidant and Anti-Microbial Activity in Quercus cerris Bark Extracts.

Adrian Nisca,Cristina Nicoleta Ciurea,Daniela-Lucia Muntean,Andrei Mocan,Cadmiel Moldovan,Anca Delia Mare,Ruxandra Ștefănescu,Corneliu Tanase,Adrian Man

doi:10.3390/plants11030240

Abstract

The species of the Quercus genus, including Quercus cerris L., are widely used and their wood represents a valuable material utilized for various purposes. The intense industrial processing of wood results in a considerable amount of poorly-used natural by-products, such as bark, and a loss of potentially useful raw materials. Thus, the aim of our study was to evaluate the phytochemical characteristics and potential biological activities of Quercus cerris bark extracts obtained by optimizing the parameters of microwave assisted extraction (MAE). The optimum conditions for MAE were determined using a design of experiments (DoE) model, which set the experimental variables (irradiation time and microwave power) and their values. Aqueous and hydroalcoholic extractions were performed and the optimum parameters of extraction were determined for both solvents. The total polyphenolic and tannin contents were determined. The biological activities representative of antioxidant capacity were determined using two free radical scavenging methods, the DPPH and ABTS methods, and the antibacterial activity was assessed with the microdilution method. The results showed different optimal extraction conditions for aqueous (30 min at 850 W) and hydroalcoholic (18 min at 650 W) extracts. A higher yield of total polyphenols was observed in the hydroalcoholic bark extract (403.73 ± 7.35 mg gallic acid equivalents/g dried weight); however a lower level of tannins was registered in comparison to the aqueous extract. In addition, both extracts exerted high antioxidant activities, with the aqueous extracts having a stronger inhibitory effect against the DPPH radical. Moreover, the extracts exhibited antibacterial activity against the tested bacterial strains, especially against the Gram-positive strains and Klebsiella pneumoniae, with the hydroalcoholic extracts being more efficient overall. To conclude, the optimized MAE was an efficient method to extract phytochemical compounds with potential biological effects from Quercus cerris bark.

Highlights

The Quercus genus is one of the most important genera of the Fagaceae family, as it comprises over 600 species, mainly trees, that are widely spread in the Northern Hemisphere, mainly in Europe, Asia, and the Americas [1,2]
The aim of our study was to evaluate the phytochemical characteristics and potential biological activities of Quercus cerris bark extracts obtained by optimizing the parameters of microwave assisted extraction (MAE)
Turkey oak (Quercus cerris L.) represents one of the native oak species found in central Europe [4], and one of the oak species found in all the regions of Romania but mainly in the western part of the country [9,10,11]

Summary

Introduction

The Quercus genus is one of the most important genera of the Fagaceae family, as it comprises over 600 species, mainly trees, that are widely spread in the Northern Hemisphere, mainly in Europe, Asia, and the Americas (especially North America) [1,2]. Recent research focused on the analysis of different types of oak barks suggested that this so-called natural waste might be an important source of bioactive compounds, mainly polyphenols, such as phenolic acids, tannins, and flavonoids [5]. In one study, Sen et al determined the total phenolic acids, condensed tannins, and flavonoid contents in hydroalcoholic extracts obtained from the cork and phloem fractions of Q. cerris. The same study indicated the antioxidant capacity of the extracts [15] Phenolic compounds, such as vanillic, homovanillic, and isovanillic acids, vanillin, and syringic acid, were identified in the hydroalcoholic extracts obtained from the wood of Q. cerris. These extracts exerted antioxidant activities [16]. Aqueous and methanolic Q. cerris bark extracts have been shown to inhibit the proliferation of the Hep-2 (human larynx carcinoma) cell line [17]

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